Thunder, the sound that follows lightning, comes from rapid air expansion around the lightning bolt. The heat from lightning causes the air around the bolt to reach temperatures of over 40,000 degrees Fahrenheit rapidly. The heated air compresses then explodes outwards, forming a shock wave and creating a loud noise.
Lightning bolts are most often vertical. Their vertical form causes individuals to hear the sound waves closest to the ground first followed by the sound waves from higher up. Rumbling sounds happen when sound waves from different parts of a jagged or forked lightning bolt reach the ears in a series.